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Balancing Total Estimated Recoverable Storage and Sustainability - - PowerPoint PPT Presentation
Balancing Total Estimated Recoverable Storage and Sustainability - - PowerPoint PPT Presentation
Balancing Total Estimated Recoverable Storage and Sustainability Sustainability Wade A. Oliver, P.G. Texas Alliance of Groundwater Districts Groundwater Summit August 27, 2015 The 9 Factors Districts Shall Consider When Adopting
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Definitions
Total Estimated Recoverable Storage—The estimated amount of groundwater within an aquifer that accounts for recovery scenarios that range between 25% and 75% of the porosity-adjusted aquifer volume Texas Administrative Code Sec. 356.10
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Types of Aquifers Unconfined Confined
Northwest Southeast Northwest Southeast Measured Water Level
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Guidance from TWDB
30 miles x 30 miles 900 square miles 576,000 acres 500 feet 15%
Typical County Example
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Guidance from TWDB
30 miles x 30 miles 900 square miles 576,000 acres 500 feet 15%
Typical County Example
43.2 million acre-feet =
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Guidance from TWDB
30 miles x 30 miles 900 square miles 576,000 acres 500 feet 0.15
Typical County Example
43.2 million acre-feet = 500 feet 0.0001 0.03 million acre-feet =
Unconfined Portion: Specific Yield Confined Portion: Storativity
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MAGs and Storage by GMA
GMA Total Estimated Storage (million acre-feet) MAG in 2060 (million acre-feet) Total MAG over 50 Years (million acre-feet) Total MAG as Percent of Storage 1 588 2.27 150 25.0% 2 968 1.344 94 9.7% 3 476 0.461 23 4.8% 4 160 0.207 10 6.3% 5 NA NA NA NA 6 180 0.422 22 12.2% 7 447 0.648 33 7.4% 7 447 0.648 33 7.4% 8 1,628 0.386 19 1.2% 9 33 0.096 5 15.2% 10 46 0.1 5 10.9% 11 2,488 0.543 27 1.1% 12 1,380 0.337 15 1.1% 13 2,756 0.485 24 0.9% 14 3,085 0.907 47 1.5% 15 443 0.488 24 6.0% 16 2,205 0.358 18 0.8% Total 16,883 9.052 516 3.1%
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Implications
Water demand in Texas
– 20 million acre-feet between 2020 and 2070 – At 25% recoverability, do we have enough groundwater to meet all demands for 200 years even if it doesn’t rain again? – “If it sounds too good to be true…”
No consideration given to: No consideration given to:
– Aquifer water quality – Water levels dropping below pumps – Land surface subsidence – Degradation of water quality – Changes to surface water-groundwater interaction – Recharge – Practicality/economics of development
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Unconfined vs. Confined Storage
Takeaway: In theory, each foot of drawdown yields much more water when an aquifer is unconfined than when it is confined.
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From Heath (1983)
Specific Yield Storativity
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Aquifer Lithology
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Drawdown vs. Storage Volume in an Example Confined Aquifer
Exact shape of curve is aquifer specific and depends on initial water levels, aquifer thickness, and storage properties (storativity and specific yield). Idealized curve developed using a 500 ft thick aquifer with 500 feet of confined head. Storativity set to 0.0001 and specific yield (Sy) is varied.
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Balancing Test
Paraphrased Factors in Texas Water Code Sec. 36.108(d-2) : “The desired future condition proposed under Subsection (d) must provide a balance between the highest practicable level of groundwater production and the conservation, preservation, protection, recharging, and prevention of waste of groundwater protection, recharging, and prevention of waste of groundwater and control of subsidence in the management area”
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Balancing Test
Total Estimated Recoverable Storage Highest Practicable Conservation, Preservation, etc. Less Pumping More Pumping Storage Modeled Available Groundwater Must Fall in this Range
Highest Practicable Total Estimated Recoverable Storage
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Balancing Test
Aquifer uses or conditions Water supply needs and Environmental Impacts Subsidence
Total Estimated Recoverable Storage Highest Practicable Conservation, Preservation, etc.
Water supply needs and management strategies Subsidence Impacts Socioeconomic Impacts Private Property Rights Feasibility of achievement Socioeconomic Impacts Private Property Rights Hydrological Conditions Hydrological Conditions
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In Conclusion
- As calculated, TERS represents the approximate
fraction of total storage in the aquifer that is in the water-producing zones (e.g. sands), not what is “recoverable”.
- TERS is a simple volumetric calculation that does
not account for many important factors that limit not account for many important factors that limit groundwater production
- With few exceptions, TERS is far greater than the
highest practicable level of groundwater production and is not a useful tool for the planning and management of aquifers.
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